half pressure on one zone unless i turn on another zone simultaneously.

I have three active zones on my sprinkler system (used to have four, disconnected it because it was a drip line, did it last year, no problems with it since then). My first zone, zone #1, is only getting half pressure on all the heads on the line off the controller. I opened this zone manually by hand, both via vent and unscrewing the solenoid, and I still only get about half pressure at all the heads. So the solenoid I'm assuming is fine. Then, while zone #1 is on, I manually open zone #2 (or #3, same results) simultaneously and all of a sudden zone #1 has full pressure. So I automatically think its a valve problem... I don't know how but I can get full pressure on zone #1, so it can't be an obstruction or pinched pipes somewhere right?

Well I replace out the valve (irritrol, all of them are irritrols), and the problem still continues; I still only have half pressure on zone #1, unless I simulatenously and manually open valves for zone #2 or zone #3.

Here's an interesting side note:

I brought the whole system online about six weeks ago after a good winterization. Turned it on, no leaks, full pressure on every zone, good to go, so I assumed I winterized it correctly. Well, I planned on expanding my driveway, so I capped one of the stem lines off of zone #1. I didn't rip the whole line out to the t connection with the main line, just capped it, was gonna worry about it later. I did this about 2 weeks after bringing the whole system back on. The only way I caught zone #1 not getting full pressure is because there's a dead spot forming in the middle of my lawn (I have the controller set to go off at about 5:00 AM). So now I'm beginning to think that my system was fine, and I screwed it up by capping that one head. I checked that capped head, it is NOT leaking water. I even unscrewed the cap a little to see if maybe there was an air blockage in the system somehow... I got full pressure of water soaking me.

Can anybody explain to me what might be happening? Why only half pressure unless I turn another zone on simultaneously? Is it because of this capped end and maybe somehow it has trapped air in it and pushing back on the line for zone #1?

I believe there's a problem with the new valve as well.
You could hook the pressure line directly up to the lateral line bypassing the suspected problem valve all together. I know that sounds drastic but it would narrow your search. Maybe install a ball valve in place of the electric valve just as a test...

Yup it was that capped head. I replaced the cap with a sprinkler head, got full pressure and it was all good to. I don't really wanna rip it out, so I'm currently engineering a drip line off of it instead. Hey, I really appreciate all the help and suggestions guys. Seriously, I posted on several forums, and only you guys gave me the response and suggestions. God bless y'all and enjoy your summers.